The utility of expanded perlite as a loose-fill insulation, e.g., block and cavity fillers; aggregates in plaster and wallboard; aggregates in lightweight concrete, e.g., roof decks and prefabricated structural panels; a filtration medium; a soil conditioner; a paint filler; a constituent of oil-well drilling muds and concretes; and inert packing materials is widely known. See Tver and Bolz, Encyclopedic Dictionary of Industrial Technology, Materials, Processes and Equipment, Chapman and Hall, 1984, pages 226-227. However, expanded perlite, which is generally formed by the conversion of contained water in crushed perlite into steam by abrupt heating, absorbs moisture readily and is readily dispersible in an aqueous medium. These properties greatly restrict the commercial applications and the uses of expanded perlite.
Porous inorganic substrates, e.g. insulators such as diatomaceous earth brick, electrical insulators, porcelain spark plugs, and various building compositions have been shown to be rendered water repellent by silanes and especially by alkoxysilanes. However, the silane compositions currently utilized for such purposes employ various organic solvents such as alcohol, e.g, Seiler, U.S. Pat. Nos. 3,772,065, and Brown et al., 4,342,796; or hydrocarbons, e.g., Linn, 4,525,213, limiting the production and the use of such water repellent products due to the toxicity, the flammability and the like of the solvents.
Therefore, water repellent porous inorganic substrates produced by treatment with aqueous silane compositions which are of low toxicity and non-flammable have become important. See, Puhringer, U.S. Pat. Nos. 4,433,013; Schmidt, 4,517,375; and DePasquale and Wilson, 4,648,904. Such compositions can have an important drawback, however, and that is that the pH of the aqueous silane component may tend to drift. The silane will then react with water and polymerize. This reduces the hydrophobic properties of the resultant insulation or building component compositions by reducing the content of he active, water-repellent ingredient. Moreover, the pH of the system may shift when additives are included such as a biocide, which is commonly added to retard growth of fungi and the like. Although the stability of the water-containing emulsions can be enhanced to some extent by refrigerating them, or the problem of reduced efficacy can be avoided by using the emulsions soon after preparation, both expedients make production of these water repellent insulation and building component compositions costly and may in some cases actually encourage the use of the toxic and flammable above-mentioned solvent borne, non-aqueous compositions.
Copending application U.S. Ser. No. 07/189,146, filed May 2, 1988, discloses that by selecting appropriate silanes (largely water insoluble) and appropriate emulsifiers, water-based, normally hydrolyzable silane emulsions may be prepared that are hydrolytically stable if the pH is maintained in a predetermined pH stable range, typically 6-8, with a buffering compound, comprising particularly a substance or a combination of substances, which when dissolved in water produces a solution which resists a change in its hydrogen ion concentration upon the addition of acid or alkali, and that these buffered aqueous systems may be used to increase the resistance to penetration by aqueous medium of a porous substrate.
It has now been discovered that expanded perlite can be rendered hydrophobic by treatment with buffered silane emulsions and that the resultant hydrophobic expanded perlite compositions exhibit improved wet-out properties in organic resins, improved hydrolytic and storage stability, improved performance as a filter aid, reduced water absorption and reduced solubility in aqueous media.